Encyclopedia of radio electronics and electrical engineering / Civil radio communications
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In the broadcasting range of 88 ... 108 MHz (VHF-2 or, as it is also called, the FM band), many radio stations are currently operating (in large cities their number reaches several dozen). Unfortunately, domestic radio receivers of the past years are not designed to receive transmissions in this range. A possible way out is to manufacture a VHF set-top box (in fact, a high-frequency FM path) or (if the receiver has a VHF-1 range) a converter, which not everyone can do. But there is another way, which can be used even by a person who is little familiar with radio reception equipment.
Fig. 1
On sale there are many miniature imported radios and so-called MP3 players, which provide for the reception of radio stations in the VHF-2 range. If the output of such a device, designed for connecting headphones, is connected to the input of an AF amplifier of any domestic receiver, then loud-speaking reception in the VHF-2 range will become possible. In other words, all that needs to be done is to make a connecting cable with a plug for connecting to a miniature device on one side and a connector for connecting to the input of the AF amplifier of the old receiver on the other.
A diagram of such a cable is shown in fig. 1. For reception in the VHF-2 range, a small-sized M-820 receiver (made in China) was used along with a domestic VEF-12 receiver, which has a socket for connecting a tape recorder. The absence of modifications to the receivers allows, as before, to use each of them separately. In addition, according to the proposed scheme, any cell phone that provides for the reception of VHF radio stations can be used as a signal source.
Fig. 2
If the old receiver does not have an outlet for connecting an external signal source, it is not difficult to introduce one. Usually, the variable resistor of the volume control (R1 in Fig. 2) is connected as a divider of the output voltage of the detector: one of its outputs is connected to the detector, the second - to the common wire, and the output of the engine - to the input of the AF amplifier.
When introducing an XS1 socket (it can be of any type) to connect a receiver with a VHF band, the output of the variable resistor R1 connected to the detector output is disconnected from the latter and connected to one of the socket sockets (the second one is connected to a common wire) and the closing contact of the additional switch SA1 (also any type) . Its opening contact is connected to a common wire through a resistor R2 of the same resistance as the variable one, and the movable contact is connected to the detector output.
After such refinement, in position "1" of the switch, the input of the AF amplifier will be disconnected from the output of the detector, which will eliminate interference with reception in the VHF band from the receiving path of the old receiver, and in position "2" - connect to it, allowing reception, as well as before remodeling.
Author: Yu. Belaventsev
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